1. Field of the Invention
The disclosure relates to the field of electronic communications. More particularly, the disclosure relates to the field of configuring and interfacing with access channels in communication systems.
2. Description of Related Art
During the last decade many cellular communications standards have selected orthogonal and non-orthogonal Code Division Multiple Access (CDMA) physical layer interfaces.
In the context of many to one communications where several users try to send information to a central receiver, non-orthogonal CDMA has been the option of choice. The reverse links of cdma2000 and WCDMA are good examples of this.
The fundamental characteristic of a non-orthogonal CDMA channel is that it is self-interference limited. The degradation in the communication between a user and the central receiver is primarily due to other users of the system that are simultaneously accessing the channel at the same frequency band. Each concurrent transmitter is only distinguishable by the code it utilizes. Furthermore, for the system to work, the energy that is present in the medium due to other users transmissions needs to have practically the same statistical properties as white noise. It is this randomness that allows several users to successfully transmit information at the same time in the same frequency band as long as the number of simultaneous users does not exceed some maximum N. Typically, a different transmission code is assigned to each user by the central entity. The special properties of these codes assure the desired characteristics of the interference.
In a circuit-switched CDMA channel like the reverse link of cdma2000, the actual number of users U present in the system is of the same order of magnitude as the number N of maximum allowed simultaneous users for successful transmission. This connection oriented configuration is well suited for voice-like applications with steady traffic needs. For instance, a typical voice encoder produces 192 bits every 20 milliseconds. Furthermore, the transmission of frames is arranged in a way such that once the receiver has acquired a particular user it knows exactly when to expect the next information frame. Conceptually the receiver is comprised of U parallel receivers, each acting in one of the codes. For typical cdma2000 deployments, U is approximately 60, which may be implemented in a relatively low complexity receiver.
For a different type of user traffic, like web browsing, the per user utilization of the channel is much more sporadic, so that the total number of users U that a system can effectively support is much larger than the allowed number of simultaneous transmissions N. Some systems are being proposed where N˜30 and U˜15000. Furthermore, the sparse nature of the traffic suggests a non connection oriented Aloha type access protocol. In an Aloha based access channel, each user accesses the channel whenever the user has data to transmit. If multiple users attempt to concurrently access the same channel space, a collision may occur and both transmissions may be unsuccessful.
In the Aloha based access channel, the time of arrival of the information frames is unknown at the receiver with a probability distribution that is flat over time. This adds an extra dimension (arrival time) to the demodulator complexity since every possible transmission code has to be continuously checked for the arrival of packets. In practical terms it is much more complex to demodulate a signal transmitted using a given code when the arrival of the signal is unknown. The individual demodulators that are needed for the CDMA-Aloha channel are orders of magnitude more complex than the ones mentioned above for the connection oriented protocols.
It is in principle undesirable, in terms of receiver complexity, to assign 15000 different codes and have 15000 parallel demodulators. One possible approach is to have a smaller set of codes C<U from where the users randomly choose one every time they want to start a transmission. Limiting the number of access codes increases the probability that collisions can occur.
While simultaneous transmissions are allowed, two different transmitters using the same code and arriving at the receiver at the same time will not interfere randomly with each other. The mixing of the information symbols on the same code at the same time and frequency band will likely cause the loss of both packets. This can be solved by having a sufficiently large set of codes C such that collisions are very unlikely. However, receiver complexity increases with an increase in the number of available codes C.
It is desirable to have an access channel configuration and protocol within a communication system that allows for a large number of active, intermittent, users while reducing the probability of collision for data transmissions from distinct users, and maintaining or reducing the complexity of an associated receiver.